This invention relates to loudspeaker enclosures, and more particularly to vented or ported loudspeaker enclosures.
The majority of loudspeaker transducers designed for use in air can be described as a piston attached to a linear motor system. An alternating electrical signal fed into the motor causes the piston or diaphragm to vibrate accordingly, so creating sound waves in the surrounding air.
As the diaphragm moves in one sense so the air on one side of the diaphragm is compressed while the air on the other side is rarefied, and vice versa. Thus, the sound waves emitted from the two sides of the diaphragm are of opposite phase. In order to prevent cancellation between the two, the transducer is normally mounted in some kind of enclosure which contains the radiation from one side of the driver. Such enclosures may be sealed or may be vented by way of a port, amongst other configurations.
At low frequencies the enclosed volume of air behaves as a simple compliance but standing waves will be excited within the enclosure at higher frequencies where the wavelengths are similar in scale to the enclosure dimensions. These resonances may then be heard superimposed on the output from the front side of the diaphragm, to the detriment of the overall fidelity of the reproduction.
The low frequency output of a loudspeaker driver may advantageously be reinforced at low frequencies by the addition of a port connecting the inside of the enclosure to the air outside. The combination of the mass of air in the port, coupled to the enclosed air spring or compliance, forms a Helmholtz resonator which would normally be tuned to a frequency somewhat lower than the low frequency resonance of the driver in an equivalent sealed enclosure, thereby extending the low frequency extension of the system. However, this arrangement tends to exacerbate the leakage of any internal standing waves to the outside world.
Absorbent material, including fibrous tangles such as long fibre wool, may be used to attenuate standing waves but does not eliminate them. Also, when such material is used in conjunction with a vented system there is a tendency for the quality of the port resonance to be deleteriously affected as the damping effect of the fibre also acts as a loss in the Helmholtz resonator.
It is an object of the invention to provide a loudspeaker enclosure that is vented or ported and which includes means for controlling standing waves.